Antiestrogens are the most frequently used drugs for breast cancer (BC) treatment. However, most patients develop resistance. Growth factors (GFs) play a critical role in this process. We have shown that several GFs modulate the expression/activity of ER-a in MCF-7 cells, mediated by the phosphatidylinositol 3-kinase (PI 3 K)/Akt1 pathway and erbB2 is necessary for their effect. Stable transfection of cells with a dominant negative Akt1 (K179M) or the R25C-Akt1 mutant block the effects of estradiol and GFs on ER-a activity and on in vitro and in vivo cell growth, while stable transfection of cells with a constitutively active Akt1 mutant (myr) mimics the effects of estradiol in vitro. In the presence of myr-Akt1, tamoxifen can only partially block in vitro cell growth. To define the role of erbB2 and Akt1 in the development of hormone resistance, we used the in vitro model system MCF-7/ LCC, in which sequential in vivo selection of MCF-7 cells (LCC1) with tamoxifen (LCC2) or ICI 182,780 (LCC9) led to endocrine resistance. In all 3 LCC cells, expression of erbB2 was elevated compared to MCF-7 cells and celf growth was blocked by a selective erbB2 and a PI3-K inhibitor. Based on these results, we developed a molecular model of cross-talk between ER-a and erbB receptors that converges into the PI 3-K/Akt pathway. Estradiol, bound to membrane ER-a interacts with and activates the ErbB2* ErbBS heterodimer. ErbBS binds to PI 3-K, which activates Akt1. Akt1 phosphorylates ER-a thereby altering its transcriptional activity, cell proliferation, and antiestrogen response. Therefore, a subset of 18 genes: ANXA6, ARC, EIF5A, ASCL1, HES1, EGR1, EGR3, FOS, MYC, MYB, SMAD7, TGF-b1, QRT1, TOP2A, TXNIP, CYR61, and CTGF (identified by gene array experiments) that are regulated by both estrogen and Akt1 may contribute to hormone resistance. The aims are: 1) to identify estrogen and Akt1- regulated genes after tamoxifen treatment in MCF-7 cells (parental or stably transfected with myr, K179M, and R25C), and 2) to define which of these genes are aberrantly expressed in patients treated with tamoxifen (tamoxifen-resistant versus tamoxifen-sensitive tissue). The identification and characterization of new downstream targets of estradiol and Akt1 may represent important effectors of antiestrogen resistance in hormone-dependent BC, leading to the discovery of new molecular therapeutic targets and/or predictive clinical markers. [unreadable] [unreadable] [unreadable]